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REVISION: REFRACTION & DIFFRACTION
11 JUNE 2013
Lesson Description
In this lesson, we revise:
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

what happens when light is refracted and undergoes total internal reflection
calculations when light is reflected and refracted
what happens when waves are diffracted
Key Concepts
Important facts about light
Light travels in straight lines, which explains why we can’t see around corners!
As far as we know nothing travels faster than light since the speed of light is
. Sound
-1
travels at approximately 340m.s , which means that light is approximately a million times faster than
sound.
When light hits an object, one of three things can happen:
1. Absorbed
2. Reflected
3. Refracted
Reflection of light
When light shines on a shiny surface such a mirror, the light is reflected back, away from the surface.
This reflection is occurs in a regular, specific way.
Important Terminology

The normal is an imaginary line perpendicular (at 90 ) to
the surface. All angles are measured from the normal.

The incident ray is the original light ray

The reflected ray is the ray that is reflected off the
surface.

Θi is the angle of incidence. It is the angle between the
normal the incident light ray
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Θr is the angle of reflection. It is the angle between the
normal the reflected light ray
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Laws of Reflection
1. The incident ray, the reflected ray and the normal are all in the same plane
2. The angle of incidence is equal to the angle of reflection.
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Refraction of light through a rectangular prism
When light passes from a more optically dense medium to a less optically dense medium, it is
refracted away from the normal.
When light travels from a less optically dense medium to a more optically dense medium, it is
refracted towards the normal.
The incident ray and emergent rays are parallel
The relationship between the angles is:

1 angle of incidence = 2
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2 angle of incidence = 1 angle of refraction
st
nd
nd
st
angle of refraction;
Refraction of light through a prism:
The angle of deviation is measured between the original direction of the incident ray and the direction
of the emergent ray
The incident ray and emergent rays are not parallel because the sides of the prisms are not parallel.
The emergent ray is bent towards the sides of the prism. (Away from the normal)
As light moves from medium to another it is refracted. This refraction occurs because the speed of
the light changes and causes the light to change direction (if it hits the boundary at an angle greater
o
than 0 )
To describe the extent to which the speed of light in a medium differs from that in a vacuum we use a
quantity called the refractive index.
Refractive index: The refractive index n of a material is the ratio of the speed c of light in a vacuum
to the speed v of light in the material.
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Refractive index of some common materials
Substance
Refractive index
Air
1,0
Water
1,33
Perspex
1,5
Glass
About 1,5
Diamond
2,4
Note: The speed of light in a vacuum and air is significantly close
enough for the refractive index of air to be the same as the refractive
index for a vacuum, i.e. I,0.
When light travels from a medium where the refractive index is
smaller (i.e. air) into a medium where it is larger (i.e) glass, the light is
refracted towards the normal. The amount of refraction that occurs is
dependent on the refractive index of the materials involved and the
angle of incidence. The relation between theses quantities is known
as Snell’s law of refraction.
Snell’s Law of Refraction: When light travels from a material with
refractive index n1 into a material with refractive index n2, the refracted ray, the incident ray and the
normal to the interface between the materials all lie in the same plane. The angle of refraction, θ r, is
related to the angle of incidence, θi, by:
Total Internal Reflection
Consider a ray of light moving from water to air. It will be refracted away from the normal
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There is a certain angle at which the refracted ray is refracted at 90 , i.e. along the water surface. In
this case we say that the angle of incidence is equal to the critical angle.
Any ray of light that is incident at greater than the critical angle, cannot be refracted into the air. It is
refracted back into the water. This phenomenon is known as total internal refraction.
The conditions for total internal refraction to take place:
1. Light ray must move from greater optical density to a medium with lower optical density
2. Angle of incidence must be greater than the critical angle
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Diffraction: Huygen’s Principle
A wavefront is an imaginary line joining all the points in a wave that are in phase. That is all the
crests could be joined together to form a wavefront.
Huygen’s Principle states: All points on a wavefront act like a source. Each one of these point
sources (secondary sources) produces small circular waves moving forwards with the same speed as
the waves. The new wavefront is obtained by drawing a tangent to all the new little wavefronts.
Diffraction: Water Waves
When straight water waves move past a barrier the waves bend around the barrier.
Each wavefront is a source of secondary sources. As
the wave moves past the barrier each secondary
source creates another secondary circular wavefront.
These new wavefronts are composed of new sources,
which in turn move forward in a circular pattern and
thus the wave ‘bends’ past the barrier.
The amount of diffraction (bending) of the wave
depends on the width of the slit in the barrier.
Diffraction: Light Waves
Just as with water waves, light can be diffracted.
When the light is diffracted through a single slit barrier
a unique pattern can be observed on a screen. There
will be a bright central band with alternating dark and
bright bands on either side of the central band. The
bright bands become less intense the further they are
from the central band.
The bright bands are a result of constructive
interference from the secondary wavefronts formed
when the wave moves through the slit.
The dark bands are a result of destructive
interference from the secondary wavefronts formed
when the wave moves through the slit.
The amount of diffraction is dependent on the width of the slit. The narrower the slit the greater the
diffraction and thus the wider the central band. Maximum diffraction happens when the slit width is
equal to the wavelength of the wave passing through the slit.
When the slit is wider, then the diffraction is less and the central band is narrower.
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Questions
Question 1
Light is passed from air into a rectangular glass prism.
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a.) What angle of incidence would give an angle of refraction of 10 ?
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b.) What angle of refraction would you get for an angle of incidence of 36 ?
Question 2
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A ray of light strikes a plane mirror so that the angle of incidence is 30 . What will the angle between
the mirror and the reflected ray be? Substantiate your answer with a diagram
Question 3
State the three things that could happen to a ray of light when it falls on an object.
Question 4
A ray of light is shone through a sapphire. Calculate the speed of light through the sapphire.
Question 5
If light moves from glass to a diamond, will the speed increase, decrease or stay the same? Give a
reason for your answer by referring to the refractive index and optical density of each medium.
Links
1. Reflection of light: http://www.youtube.com/watch?v=81u7MGF7ev4
2. Reflection and refraction of coloured light in water:
http://www.youtube.com/watch?v=2kBOqfS0nmE
3. Refraction: http://www.youtube.com/watch?v=xwzw5GhpC_0
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